1. Field of the Invention
This invention relates to a coating for a metallic substrate. More particularly, a transparent coating is deposited on surfaces of that substrate to improve adhesion of the substrate to a polymer resin.
2. Description of the Related Art
A tenacious bond between a metal and a polymer is desired in many electrical and electronic assemblies. As one example, an electrical connector may include a metallic jack surrounded by a molded plastic insulating sheath. As a second example, the inner leads of a metallic leadframe may be encapsulated in a polymer molding resin in the assembly of an electronic package to house an integrated circuit. In both examples, as well as other applications, delamination at the metal to polymer interface allows the ingress of moisture and other contaminants that detrimentally impacts the operating performance of the assembly.
The adhesion between a polymer resin and a metallic substrate is improved by coating the metallic substrate with a second metal that forms a more tenacious bond to the polymer resin. Suitable coatings include refractory oxide forming metals such as nickel and nickel alloys or intermetallic forming coatings such as tin on a copper substrate. These coatings, as well as others, are disclosed in U.S. Pat. No. 4,888,449 to Crane et al, which is incorporated by reference in its entirety herein.
Anodization of aluminum alloy substrates enhances the strength of a polymer bond as disclosed in U.S. Pat. No. 4,939,316 to Mahulikar et al. Other satisfactory coatings include oxidation resistant materials having a thickness of 300-5,000 angstroms as disclosed in U.S. Pat. No. 5,192,995 to Yamazaki et al.
U.S. Pat. No. 5,252,855 to Ogawa et al discloses improving the adhesion between a copper base metallic substrate and a polymer resin by anodically oxidizing the substrate to form an aggregate of acicular crystals.
The ideal coating material is transparent and does not change the cosmetic appearance of the metallic substrate. Portions of the substrate may be soldered and the coating should be either solderable or easily removed from the substrate. If the coating is removed, the surface finish of the substrate should not be changed.
Other desired properties of the coating include corrosion resistance, oxidation resistance and enhanced adhesion to the polymer. In addition, the coating should be capable of application to any metallic substrate.
None of the prior art coatings satisfy all of these requirements. There remains a need for an improved coating for metallic substrates.